Gravitational perturbations of a Kerr black hole in f(R)f(R) gravity

Modified theories of gravity are often built such that they contain general relativity as a limiting case. This inclusion property implies that the Kerr metric is common to many families of theories. For example, all analytic $f(R)$ theories with vanishing constant term admit the Kerr solution. In any given theory, however, the response of the gravitational field to astrophysical disturbances is tied to the structure of the field equations. As such, even if black holes are Kerr, the underlying theory can, in principle, be probed through gravitational distortions. In this paper, we study linear perturbations of a Kerr black hole in $f(R)$ gravity using the Newman-Penrose formalism. We show that, as in general relativity, the equations governing the perturbed metric, which depend on the quadratic term of the function $f$, completely decouple.Comment: 8 pages. Accepted for publication in Phys. Rev.

Evolutionary implications of a magnetar interpretation for GLEAM-X J162759.5-523504.3

The radio pulsar GLEAM-X J162759.5-523504.3 has an extremely long spin period (P = 1091.17\, \mbox{s}), and yet seemingly continues to spin down rapidly (\dot{P} < 1.2 \times 10^{-9}\, \mbox{ss}^{-1}). The magnetic field strength that is implied, if the source is a neutron star undergoing magnetic dipole braking, could exceed 10^{16}\,\mbox{G}. This object may therefore be the most magnetised neutron star observed to date. In this paper, a critical analysis of a magnetar interpretation for the source is provided. (i) A minimum polar magnetic field strength of B \sim 5 \times 10^{15}\,\mbox{G} appears to be necessary for the star to activate as a radio pulsar, based on conventional `death valley' assumptions. (ii) Back-extrapolation from magnetic braking and Hall-plastic-Ohm decay suggests that a large angular momentum reservoir was available at birth to support intense field amplification. (iii) The observational absence of X-rays constrains the star's field strength and age, as the competition between heating from field decay and Urca cooling implies a surface luminosity as a function of time. If the object is an isolated, young (\sim 10\, \mbox{kyr}) magnetar with a present-day field strength of B \gtrsim 10^{16}\,\mbox{G}, the upper limit (\approx 10^{30}\, \mbox{erg s}^{-1}) set on its thermal luminosity suggests it is cooling via a direct Urca mechanism.Comment: 12 pages, 6 figures. Accepted for publication in MNRA

Spatial Structure of Mountain Forests of the Lake Baikal Southwestern Coast

Spatial structure of mountain forests of the Baikal region is presented through geobotanical mapping Correlated analysis of the vegetation cover structure and data of landscape investigations was done The study area is characterized by a high natural diversity of plant communities with dominating forests reflecting the ecotopes differentiation in mountain conditions as well as by the contact of regional physical geographical structures and experiencing anthropogenic impacts The map legend conveys the identified spectrum of forests diversity Area is provided for the integrated stability areas of plant communities Forest stability is regarded as ability of plant communities to retain their phytocenotic structure An expert assessment of stability is made This territory is notable for dominance of stable and moderately stable forest communities The modern mosaicdispersed distribution of communities of equifinal successional stages gives evidence of the current favorable conditions for preservation of dark coniferous forest

Measuring spin in coalescing binaries of neutron stars showing double precursors

Gamma-ray bursts resulting from binary neutron-star mergers are sometimes preceded by precursor flares. These harbingers may be ignited by quasi-normal modes, excited by orbital resonances, shattering the stellar crust of one of the inspiralling stars up to $\gtrsim10$ seconds before coalescence. In the rare case that a system displays two precursors, successive overtones of either interface- or $g$-modes may be responsible for the overstrainings. Since the free-mode frequencies of these overtones have an almost constant ratio, and the inertial-frame frequencies for rotating stars are shifted relative to static ones, the spin frequency of the flaring component can be constrained as a function of the equation of state, the binary mass ratio, the mode quantum numbers, and the spin-orbit misalignment angle. As a demonstration of the method, we find that the precursors of GRB090510 hint at a spin frequency range of $2 \lesssim \nu_{\star}/\text{Hz} \lesssim 20$ for the shattering star if we allow for an arbitrary misalignment angle, assuming $\ell=2$ $g$-modes account for the events.Comment: 11 pages, 6 figures, 2 tables, with an appendix containing 1 figur